Bursts of vegetative growth often compete with the source-sink relationships between the vegetative parts and the reproductive organs of higher plants. Those skilled in the art have often turned to Gibberellic acid transport or synthesis inhibition to control a flush of growth, i.e., plant height. While those measures are successful in controlling plant height they do not normally contribute to yield.
Auxins are known to regulate many of the physiological events in a plants life cycle. Some of these regulated events are phototropism, gravitropism, apical dominance, leaf and fruit abscission, and root initiation.
Plant shoots display positive phototropism. When plants are illuminated from one direction the shoot grows in that direction. Auxin is synthesized at the tip and translocated down along the shady side of the shoot. Auxin stimulates elongation of the cells on the shady side causing the shoot to bend toward the light.
Gravitropism is a plant growth response to gravity. Plant shoots exhibit negative gravitropism. When a plant is laid on its side a plant shoot will grow up. The opposite is true of roots. Roots show positive gravitropism because they grow down. When a root is placed on its side amyloplasts (organelles containing starch grains) settle to the bottom of cells in the root tip. Auxin sent down from the shoot arrives in the central tissues of the root tip and is then translocated back along the under side of the root. This inhibits root cell elongation on the lower side of the root. The cells at the top surface of the root elongate causing the root to grow down.
Growth of the shoot apex (terminal shoot) usually inhibits the development of the lateral buds on the stem beneath. This phenomenon is called apical dominance. If the terminal shoot of a plant is pruned the inhibition is lifted and lateral buds begin growth. The release from apical dominance enables lateral branches to develop and the plant becomes bushier. Apical dominance results from the downward transport of auxin produced in the apical meristem. In fact, if the apical meristem is removed and indole-3-acetic acid is applied to the plant's pruned apex inhibition of the lateral buds is maintained.
Auxin plays a role in the abscission of leaves and fruits. Young leaves and fruits produce auxin and they remain attached to the stem. When the level of auxin declines an abscission layer forms at the base of the petiole. Soon the petiole or fruit stalk breaks free. Fruit growers often apply auxin sprays to cut down the loss of fruit from premature dropping.
Auxins stimulate the formation of adventitious roots in many plant species. Adventitious roots grow from stems or leaves rather than from the regular root system of the plant. Horticulturists may propagate desirable plants by cutting pieces of stem and placing them base down in moist soil. Eventually adventitious roots grow out at the base of the cutting. The process can often be hastened by treating the cuttings with a solution or powder containing a natural or synthetic auxin.